ESRL Global Systems Division

Japan Earthquake Affects NOAA GPS Water Vapor Measurements

One of the 2,000 Global Positioning System (GPS)
receivers in the Japan GPS network (MIZU) is located in the city of
Oshu about 100 km (60 miles) north of Sendi Province and it bore the
brunt of the recent earthquake and subsequent tsunami. MIZU provides
real-time data that are used by GSD to "anchor" NOAA GPS-met sites in
the Western Pacific. When the earthquake struck, electrical power and
communications were interrupted and we abruptly stopped receiving data
from this site. The loss of data from MIZU did not seriously affect
the accuracy and reliability of the water vapor estimates from the
other sites in the Western Pacific Subnetwork because the data
processing system built by OAR has sufficient redundancy to deal with
the loss of data from several individual sites at the same time.

When power and communications were restored to the
site on March 16, we started to receive observations again from MIZU,
but we soon discovered that there was a serious problem: the GPS
receiver was not where it was supposed to be.

MIZU had moved approximately 4 meters to the east
of its presumably "permanent" location, along with the rest of this
part of Honshu Island. This change in location caused the GPS-Met data
processing system in Boulder to mismodel the atmosphere at every
station in the same subnetwork as MIZU. Simultaneously, quality
control software built to automatically detect errors caused by large
(greater than 10 cm or about 4 inch) changes in position stopped
sending water vapor estimates from every station in the Western
pacific subnetwork to modelers and forecasters. To restore data flow
to NCEP and NWS Western Region, we manually removed MIZU from the
network until a new "permanent" position can be determined.

Clearly, GSD has learned valuable lessons from this
tragedy: nothing is permanent and any reliable system has to deal with
change.